The reaction of methyl iodide with the Al(1 1 1) surface was studied by room-temperature scanning tunneling microscopy (STM) and by first principles calculations. It was found that at 300 K methyl iodide decomposes on the AI(I 11) surface, forming methyl (CH3) methylidyne (CH), and adsorbed iodine. Methyl groups are observed to occupy atop sites by STM. The occupation of the hollow site by methylidyne was observed in STM measurements. Total energy density functional theory calculations have shown that methyl species occupy atop Al sites (E-A = 45.3 kcal/mol), methylidyne species adsorb on fcc hollow sites (E-A = 155.0 kcal/mol), while individual iodine atoms can bind on both on-top or hollow sites with adsorption energies between 54 and 56 kcal/mol.